Due to the low cost and wide availability of sodium resources, rechargeable sodium-ion batteries (SIBs) have attracted an increasing interest as an appealing alternative to Li-ion batteries (LIBs) for electric energy storage applications. Indeed, the research and development of SIBs materials, especially cathode materials, has attracted worldwide attention. Among the reported cathode candidates, layered transition metal oxides (NaxMO2, M=Fe, Co, Ni, Mn, Cr, V, Cu, Ti . . . ) have attracted particular attention due to their potential high capacity and good rate capability. Typically, they are composed of repeating sheets of MO6 layers with Na ions being sandwiched in between the oxide layers. Typical Na layered oxides can be mainly classified into O type (On: e.g. O1, O2, and O3) and P type (Pm: e.g. P1, P2, and P3) phases depending on the surrounding Na environment and the number (m and n) of unique oxide packing layers. The letter indicates the environment where Na is located (O: octahedral; P: prismatic) and the number indicates the number of unique interlayers that are surrounded by different oxide layers. It has been reported that the electrochemical behavior of the material is heavily influenced by the structures of the phase not only because of the amount of Na in the pristine state but also due to the stability of each layer and kinetics affected by the surrounding environment of Na.
Previous research on sodium-based layered oxides have revealed that the O3-type material shows a low reversibility above 4.0 V. See Inorg. Chem. 2012, 51(11): 6211-6220. Once subjected to a charge over 4.0 V, the materials degrade considerably with large irreversible capacity loss due to increasing catalytic decomposition of the Na-based electrolytes and sodium-driven structural irreversible changes. The P2 phase was considered to be structurally more stable than the O3 phase, as the O3 phase undergoes a series of slab gliding (where one layer of the material is displaced in order from the adjacent layers) during the sodium extraction process as the strength of the Na—O bond is less than that of the Li—O bond. Because of the large fraction of unoccupied Na+ ions sites, the P2 type materials provide a lower initial sodium reservoir and an initial coulombic efficiency higher than 100%, leading to problems regarding full cell cycling and balancing. Moreover, a recent study showed that the transition from P2 to O2 is also possible if an extensive amount of Na is extracted from the host structure, implying the structural instability of P2 layered oxides at deep charge state. See Phys. Chem. Chem. Phys. 2013, 15(9): 3304-3312.